I programmed a tool to simulate combustion kinetics during firing a spud gun. During my stay in India last year I was so bored that I build a spud gun and had enough time to write this software too. I did not validate the results because of a lack of time and equipment, maybe somebody can do that for me (I am now in Germany and I can go to jail for possession of a spud gun).

For the theory of the simulation: I am a mechanical engineering student from munich with profund knowledge of thermodynamics, fluiddynamics and combustion. the theory used here is veryfied and results are reasonable.

I ran the program with the specifications of latke's L3 launcher; .3m/s front velocity.

The results did not come close to matching latke's experimental values, with energies being about 1/4 of those reported.I had some luck changing the front velocity to about 3 m/s and using pressure gain of 2.5... matched the entire curve fairly well, acctualy.

The energy/volume for a stociometric mix is nearly the same... butane is going to be giving you something like 1% more fuel/fuel-air mix volume, but is going to burn with slightly less energy/mol of O2.

Why the program changes the energy of the projectile when useing 1kg and 2kg for not a big gun? (V*V*M)/2=const (for hevy spuds) Why don't we use GGDT with 40psi, helium gas and burst disc for a valve?

@ boilingleadbath: today I am not able to open Latke's page to check his results. A pressure gain less than 4 is not realistic because the mixture will simply not ignite. Does Latke use any kind of turbulator in his combustion chamber to explain a front velocity of more than .37? Maybe a CFD (Computed Fluid Dynamic) simulation should be done on that matter*. Of course the real process will be different because of 3D flame distribution and flux towards the barrel entrance. Maybe I should do a version 2 with a simple 3D model...

@ drac: There is really no significant difference between propane and butane.

@ Rambo: The projectile energy is the integral of the pressure over the barrel length. A heavier projectile causes it to stay in the barrel longer, therefore the burnout is higher and the pressure too -> more projectile energy.

@ all friends of empirial units: I for myself am very happy with SI units especially for more complicated mathematics. If somebody wants to edit it to inches/feet/pounds/etc. I will send you the source code (it's C#).

Yeah, he has a fan in the chamber, which probably causes some degree of turbulance. Of course, I forget wheather he has the ignition behind or infront of the fan, so that may not be the cause.(turns out that it's movable, so no word on where it was during the tests)

A chamber fan adds about 40%-60% more muzzle energy acording to latke tests.